During operation of a railroad spike driving machine, when the hydraulic spike driving apparatus forces the spikes downwardly into the ties, the complementary upward force on the machine tends to lift the machine off of the rails. Accordingly, most spike driving machines include means for clampingly engaging the rails to secure the machine in place when the spikes are driven into the rails.
One form of clamping means includes a pair of jaws connected to vertically extending hydraulic cylinders supporting the jaws for vertical movement between a raised or retracted position and a lowered rail engaging position. These clamping devices also commonly include hydraulic cylinders for causing clamping engagement of the jaws with the rails. These machines have the disadvantage that they have a relatively complicated rail gripping mechanism and may require four or more hydraulic cylinders to clamp the machine against the rails.
A prior art clamping device manufactured by the assignee of the present invention includes a single horizontally extending cylinder. One end of the cylinder is connected to a clamping member which clampingly engages one rail and the other end of the cylinder is connected to a clamping member adapted to grip the other rail. The clamping members are each pivotally joined to the machine frame and include a jaw member which pivots under the head of the rail and is intended to grip the bottom surface of the head of the rail so as to restrain the machine against the rail. The jaw member is an integral part of the clamping member, and the clamping member is pivotally joined by a pivot pin to the machine frame. While this design is preferable to other prior art constructions in that it requires only a single hydraulic cylinder to cause clamping engagement with both rails, in those cases where the heads of the rails are worn down substantially, and wherein the thickness of metal between the top of the rail and the lower surface of the rail head has been substantially decreased, the clamping jaw member may not achieve a firm clamping engagement with the rail head. When the hydraulic apparatus of the spike driving machine then forces the spike downwardly into the tie, the machine will be lifted off of the rails. Repeated spike driving causes the machine to be repeatedly lifted away from the track and this can cause wear of the spike driving machine and is unacceptable to the operator.